The invention relates to a process for manufacturing a light-resisting titanium dioxide pigment suitable for matting artificial fibers; some water-soluble vanadium compound is added to a suspension of titanium dioxide pigment and precipitated on the surface of the titanium dioxide particles, whereafter the suspension is filtered, washed, thermally dried, and ground.
In almost all areas of use, such as paints, plastics, and artificial fibers, a good light-resistance is required of titanium dioxide pigments. Since titanium dioxide per se is somewhat light-sensitive, the TiO.sub.2 pigment particles have been protected by covering them with different metal hydroxide coatings. This coating takes place during the manufacture of titanium dioxide pigment at the so-called after-treatment stage, where the said metal compounds are precipitated onto the surface of calcinated pigment crystals dispersed in water. The intended use of the pigment determines which kind of after-treatment is the most advantageous. After the after-treatment the pigment suspension is usually filtered, the soluble salts are washed away from the filter cake, the pure filter cake is dried, and the dried cake is ground.
Often a change in the manufacturing technology which improves the light-resistance of TiO.sub.2 pigment in one area of use means an improvement of its light-resistance in other areas of use as well. This is, however, not always the case. Naturally each use sets several other quality requirements for titanium dioxide pigments in addition to light-resistance. At present the manufacturers of TiO.sub.2 pigments produce numerous different pigments, each with its own special properties according to its areas of use.
The artificial fiber industry uses mainly the anatase-type titanium dioxide pigment. The use of rutile is, however, increasing to some extent. One of the reasons for the popularity of anatase is that anatase particles are softer than rutile particles, and thus they will not cause as much mechanical wear and tear in the apparatuses of a fiber mill as the harder rutile particles. Anatase is, however, characterized by a very high light-sensitivity if it has not been appropriately after-treated. Neither is the light-resistance of rutile usually sufficient without an after-treatment.
A conventional after-treatment with aluminum oxide hydrate and silicic acid does not give a pigment that is used for artificial fibers a sufficient light-resistance. For this reason, it has been necessary to add to the coating various metal compounds which decrease the light-sensitivity but simultaneously worsen the brightness and tone (whiteness) of the pigment.
Previously known is, for example, a process for manufacturing a titanium dioxide pigment suitable for matting polyamide fibers, a process according to which titanium dioxide pigment is coated with manganese (II) orthophosphate so that the pigment suspended in water is treated with orthophosphoric acid or water-soluble orthophosphate and water soluble manganese (II) salt, whereafter the suspension is neutralized and the obtained pigment is filtered, washed, and dried at a low temperature, 110.degree.C at the most, and ground. The pigment must be dried at a relatively low temperature, for example, in a disperser-drier, to prevent the colorization of the pigment.
Thus, manganese compounds have been quite commonly used earlier. Manganese compounds, however, very strongly affect the tone of white TiO.sub.2 pigment. Attempts have been made to decrease this effect by precipitating the manganese into the coating in the form of manganese (II) orthophosphate, but still it has not been possible to eliminate the risk of the manganese oxidizing into strongly coloring compounds at some stage. Therefore the importance of careful drying is emphasized in this previously known process, which naturally complicates the pigment manufacture.
Antimony compounds have also been precipitated into the coating of pigment. Antimony compounds have, however, a reputation of being poisonous materials, which decreases the commercial value of antimony-containing pigments, even though the said risk of poisoning might not exist. A small amount of hydrosulphuric acid in the air causes the antimony hydroxide in the coating to convert into a sulfide and thereby to darken the pigment. Antimony compounds per se have a disadvantageous effect on the brightness of the pigment and the light-resistance is not sufficiently improved when an antimony compound is used alone.
Other metal compounds used for the same purpose are compounds of iron, chromium, cobalt, nickel, and copper; in order to obtain a sufficient light-resistance, however, these compounds must be used in such amounts that the brightness of the pigment is substantially decreased.
From U.S. Pat. application No. 2 062 137 it is also known to add to a titanium dioxide pigment some vanadium compound into a mixture, which is then calcinated at 800.degree.C minimum. When calcinated, the titanium dioxide and the vanadium compound form colorful oxides or titanates. By this process it is not possible to manufacture white titanium dioxide pigments but the end result is always a pigment which is grey or of some other color.
The object of the present invention is therefore to provide a process for manufacturing a light-resisting titanium dioxide pigment usable for matting artificial fibers, a pigment with a great brightness or whiteness.